What could cause the HTTP 500 internal server error message to occur in OWA 2003?

Based on a Microsoft knowledgebase article, there are several causes for receiving a HTTP 500 Internal Server Error. Some situations in which you might receive the error message are as follows:

• You try to access your mailbox or the public folders.
• You try to view Pretty Good Privacy (PGP)-encrypted message.
• You publish an Exchange Server 2003 Outlook Web Access site behind Microsoft Internet Security and Acceleration (ISA) Server.

For more information and steps on how to resolve this error, refer to https://support.microsoft.com/?kbid=894965.